Thrust roller bearing cage and method for manufacturing the same

ABSTRACT

A thrust roller bearing cage ( 11 ) of the present invention is included in a thrust roller bearing ( 20 ) and includes a plurality of pockets ( 21 ) accommodating rollers ( 13 ). The thrust roller bearing cage ( 11 ) includes: a radially outer area bent portion ( 41 ) formed by bending an area located radially outside the pockets ( 21 ) inward in a radial direction to a tilt angle of less than 45°; and projecting portions ( 44 ) that are formed in a tip end of the radially outer area bent portion ( 41 ) at positions aligned with the pockets ( 21 ) and project inward in the radial direction beyond radially outer edges of the pockets ( 21 ) so as to contact end faces ( 16 ) of the rollers ( 13 ) accommodated in the pockets ( 21 ).

TECHNICAL FIELD

The present invention relates to thrust roller bearing cages(hereinafter sometimes simply referred to as the “cages”) and methodsfor manufacturing the same, and more particularly relates to a thrustroller bearing cage that is manufactured by using a press and a methodfor manufacturing the same.

BACKGROUND ART

For example, thrust roller bearings that support thrust loads aresometimes placed at such locations in automatic transmissions forautomobiles, car air conditioner compressors, etc. that are subjected tothrust loads. For improved fuel efficiency and power saving, it isdesired to reduce running torque of such thrust roller bearings. Athrust roller bearing includes bearing rings arranged in the directionof the rotation axis of the thrust roller bearing, a plurality of needlerollers that roll on raceway surfaces of the bearing rings, and a cagethat retains the plurality of needle rollers. Some cages aremanufactured by bending a steel sheet and then punching out pockets thataccommodate the rollers.

A technique relating to the cages included in such thrust rollerbearings is disclosed in, e.g., Japanese Unexamined Patent PublicationNo. H10-220482 (Patent Literature 1). The thrust roller bearing cage ofPatent Literature 1 includes an annular body formed by cutting,punching, etc. and projecting portions formed in a radially outer partof the annular body. The projecting portions are formed such that thoseparts of the radially outer end face of the folded part of the annularbody which are located at the positions of the pockets project inward inthe radial direction, and the tip ends of the projecting portions faceapproximately the centers of the end faces of the rollers. PatentLiterature 1 discloses that this thrust roller bearing cage reducesrunning torque of the rollers as the tip ends of the projecting portionscontact the rollers at positions near the rotation center of therollers.

CITATION LIST Patent Literatures

Patent Literature 1: Japanese Unexamined Patent Application PublicationNo. H10-220482

SUMMARY OF INVENTION Technical Problem

In the thrust roller bearing of Patent Literature 1, the entire tip endfaces of the projecting portions contact the rollers. Since the rollersof the thrust roller bearing are subjected to a radially outwardcentrifugal force, friction that is generated between each roller andthe entire tip end face of each projecting portion, namely the portionthat contacts the roller, serves as rotational resistance. Accordingly,in the thrust roller bearing of Patent Literature 1, the rotationalresistance cannot be sufficiently reduced, and the running torque thuscannot be sufficiently reduced.

Since the projecting portions are subjected to a centrifugal force fromthe rollers, the projecting portions are required to withstand the loadfrom the rollers and to maintain their shape. Thrust roller bearings aretherefore required to have high resistance to radial loads.

In view of the above problems, it is an object of the present inventionto provide a thrust roller bearing cage that can reduce running torqueand that can maintain resistance to a radial load that is applied duringrotation of a thrust roller bearing and a method for manufacturing thesame.

Solution to Problem

A thrust roller bearing cage according to the present invention is athrust roller bearing cage included in a thrust roller bearing andincluding a plurality of pockets accommodating rollers. The thrustroller bearing cage includes: a radially outer area bent portion formedby bending an area located radially outside the pockets inward in aradial direction to a tilt angle of less than 45°; and projectingportions that are formed in a tip end of the radially outer area bentportion at positions aligned with the pockets and project inward in theradial direction beyond radially outer edges of the pockets so as tocontact end faces of the rollers accommodated in the pockets.

A method for manufacturing a thrust roller bearing cage according to thepresent invention is a method for manufacturing a thrust roller bearingcage included in a thrust roller bearing and including a plurality ofpockets accommodating rollers. The method includes the steps of:preparing a cage material that will later become the cage; forming anouter shape of the cage material so that the outer shape has portionsthat will later become projecting portions projecting inward in a radialdirection beyond radially outer edges of the pockets so as to contactend faces of the rollers accommodated in the pockets; forming thepockets in the cage material; and forming a radially outer area bentportion by bending an area of the cage material which is locatedradially outside the pockets inward in the radial direction to a tiltangle of less than 45°.

According to the thrust roller bearing cage of the present invention andthe method for manufacturing the same, the radially outer area bentportion is formed by bending the area located radially outside thepockets inward in the radial direction to a tilt angle of less than 45°.Accordingly, the projecting portions formed in the tip end of theradially outer area bent portion have a smaller contact area with therollers than in the case where the area located radially outside thepockets is not tilted (the tilt angle is 0°) as in Patent Literature 1.When the rollers are biased from the central axis of the cage toward theoutside in the radial direction by a rotational centrifugal force of therollers, friction is generated between each roller and the part of eachprojecting portion which contacts the roller. In the present invention,however, since the projecting portions have a smaller contact area withthe rollers, rotational resistance can be reduced. Running torque cantherefore be reduced.

The radially outer area bent portion is formed by bending the arealocated radially outside the pockets inward in the radial direction to atilt angle of less than 45°. Accordingly, when the projecting portionsare subjected to a rotational centrifugal force of the rollers, theprojecting portions can withstand the load from the rollers and theshape of the projecting portions tends to be maintained. Accordingly,resistance to the radial load that is applied during rotation of thethrust roller bearing can be maintained.

In the thrust roller bearing cage according to the present invention, itis preferable that the tilt angle be 25° or more and 35° or less.

In the method for manufacturing the thrust roller bearing according tothe present invention, it is preferable that, in the step of forming theradially outer area bent portion, the area of the cage material which islocated radially outside the pockets be bent inward in the radialdirection to a tilt angle of 25° or more and 35° or less.

Even if the parts of the projecting portions which contact the rollersbecome worn through the use of the thrust roller bearing cage of thepresent invention, the area of the parts of the projecting portionswhich contact the rollers increases only slightly in the case where thetilt angle is 25° or more and 35° or less. A decrease in running torquecan therefore be restrained even if the thrust roller bearing cage isused for a long period of time. Setting the tilt angle to 35° or lessmakes it easier to maintain the resistance to the radial load that isapplied during rotation of the thrust roller bearing. Setting the tiltangle to 25° or more can reduce the outside diameter of the cage.

In the thrust roller bearing cage according to the present invention, itis preferable that areas of the projecting portions which are to contactthe end faces of the rollers be subjected to a press-flattening process.

It is preferable that the method for manufacturing the thrust rollerbearing cage according to the present invention further include the stepof: press-flattening areas of the projecting portions which are tocontact the end faces of the rollers.

Since the areas of the projecting portions which are to contact the endfaces of the rollers are subjected to the press-flattening process, thiscan reduce the risk that discontinuity of a lubricant film will becaused during rotation of the bearing by the sliding motion of the endfaces of the rollers on the areas of the projecting portions whichcontact the end faces of the rollers. This improves lubricatingproperties in the contact areas and reduces what is called aggression ofthe rollers against the projecting portions of the cage. Such a thrustroller bearing cage can thus further reduce the running torque of thebearing.

As used herein, the “press-flattening process” means a process in which,in the step of forming the radially outer area bent portion, theprojecting portions are pressed outward in the radial direction by usinga radially outer surface of a die that serves as a stopper to controlthe amount of collapse, in order to smooth the rough surfaces of theprojecting portions before and after the process. Specifically, thepress-flattening process can smooth a press-sheared surface or afracture surface, which is formed in the step of forming the outershape, to arithmetic mean roughness Ra (JIS B 0601) of about 2 μm orless.

Advantageous Effects of Invention

According to the thrust roller bearing cage of the present invention andthe method for manufacturing the same, running torque can be reduced andresistance to a radial load that is applied during rotation of thethrust roller bearing can be maintained.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 shows a part of a thrust roller bearing cage according to anembodiment of the present invention.

FIG. 2 is a sectional view of the thrust roller bearing cage shown inFIG. 1.

FIG. 3 is an enlarged sectional view showing a part of the thrust rollerbearing cage shown in FIG. 2.

FIG. 4 is a flowchart illustrating representative steps of a method formanufacturing the thrust roller bearing cage according to the embodimentof the present invention.

FIG. 5 is a sectional view of a cage material after a concave and convexportions forming step.

FIG. 6 is an enlarged sectional view showing a part of the cage materialafter a pilot hole forming step.

FIG. 7 is an enlarged sectional view showing a part of the cage materialafter an outer shape forming step.

FIG. 8 shows a part of the cage material after a pocket forming step.

FIG. 9 is an enlarged sectional view showing a part of the cage materialafter the pocket forming step.

FIG. 10 is an enlarged sectional view showing a part of the cagematerial during a radially outer area bending step.

FIG. 11 is an enlarged sectional view illustrating how the radiallyouter area bending step is performed.

FIG. 12 is an enlarged sectional view illustrating how the radiallyouter area bending step is performed.

FIG. 13 is an enlarged sectional view showing a part of the cagematerial after the radially outer area bending step.

FIG. 14 is an enlarged sectional view showing a part of the cagematerial after the radially outer area bending step.

FIG. 15 is an enlarged sectional view illustrating how the radiallyouter area bending step is performed.

FIG. 16 is an enlarged sectional view illustrating how apress-flattening step is performed.

FIG. 17 is an enlarged sectional view showing the tip of a radiallyouter area bent portion after the press-flattening step.

FIG. 18 is a sectional view showing a part of a thrust roller bearingcage according to another embodiment of the present invention.

FIG. 19 shows a part of a thrust roller bearing cage according to stillanother embodiment of the present invention.

DESCRIPTION OF EMBODIMENTS

Embodiments of the present invention will be described below withreference to the drawings. In the figures described below, the same orcorresponding portions are denoted with the same reference characters,and description thereof will not be repeated.

FIG. 1 shows a part of a thrust roller bearing cage 11 according to anembodiment of the present invention. FIG. 1 shows the cage 11 as viewedin the direction of the rotation axis of the cage 11. FIG. 2 is asectional view of the thrust roller bearing cage 11 shown in FIG. 1.FIG. 2 shows the cage 11 taken along line II-II in FIG. 1. Specifically,a portion having a pocket, which will be described later, is shown insection on the right side of FIG. 2, and a portion having a pillar,which will be described later, is shown in section on the left side ofFIG. 2. FIG. 3 is an enlarged sectional view showing a part of thethrust roller bearing cage 11 shown in FIG. 2. The enlarged sectionalview in FIG. 3 shows the area indicated by III in FIG. 2. In FIGS. 2 and3, the rotation axis 12 of the cage 11 is shown by an alternate long andshort dash line. For ease of understanding, FIG. 3 shows a needle roller13 accommodated in a pocket 21 that will be described later, and a partof a pair of bearing rings 14, 15 disposed on both sides of the cage 11in the direction of the rotation axis of the cage 11. The directionperpendicular to the plane of paper of FIG. 1 and the vertical directionin FIGS. 2 and 3 are the direction of the rotation axis of the cage 11.The direction shown by arrow A₁ or its opposite direction in FIG. 1 isthe circumferential direction. For ease of understanding, the upper sidein FIGS. 2 and 3 is defined as the upper side in the axial direction.That is, the direction shown by arrow A₂ in FIGS. 2 and 3 is the upwarddirection. The lateral direction in FIGS. 2 and 3 is the radialdirection. The direction shown by arrow A₃ in FIG. 3 is the radiallyoutward direction.

First, the configuration of the thrust roller bearing cage 11 accordingto the embodiment of the present invention will be described withreference to FIGS. 1 to 3. The thrust roller bearing cage 11 accordingto the embodiment of the invention is in the shape of a disc and has athrough bore 22 extending straight through its central area in thethickness direction of the cage 11. A rotary shaft, not shown, isinserted through the through bore 22.

The cage 11 includes a pair of annular portions 23, 24 with differentdiameters, and a plurality of pillars 25 formed at intervals in thecircumferential direction so as to form the pockets 21 that accommodatethe needle rollers 13 therein and connecting the pair of annularportions 23, 24.

The pockets 21 are substantially rectangular as viewed in the axialdirection. The pockets 21 are arranged radially about the rotation axis12 of the cage 11. The pockets 21 have upper roller stoppers 26 andlower roller stoppers 27, 28 on their side wall surfaces. The upperroller stoppers 26 prevent the needle rollers 13 accommodated in thepockets 21 from falling out upward in the axial direction, while thelower roller stoppers 27, 28 prevent the needle rollers 13 accommodatedin the pockets 21 from falling out downward in the axial direction. Theupper roller stoppers 26 are formed in the middle parts in the radialdirection of the pockets 21. The lower roller stoppers 27 are formed inthe radially inner parts of the pockets 21, and the lower rollerstoppers 28 are formed in the radially outer parts of the pockets 21.The upper roller stoppers 26 and the lower roller stoppers 27, 28 areformed on the side wall surfaces located on both sides in thecircumferential direction of each pocket 21 so as to project into thepockets 21.

The needle rollers 13 are pressed into the pockets 21 to fit in thepockets 21. End faces of each needle roller 13, specifically, an outerend face 16 in the bearing and an inner end face 17 in the bearing, areflat.

The cage 11 has concave and convex portions which are formed by bendinga plate in the thickness direction thereof a few times. Specifically,the cage 11 includes four disc portions 31, 32, 33, 34 extending in theradial direction and four cylindrical portions 36, 37, 38, 39 extendingin the axial direction. The four disc portions 31 to 34 are arranged inthis order from the inside in the radial direction so that the firstdisc portion 31 has the smallest inside diameter, followed by the seconddisc portion 32, the third disc portion 33, and the fourth disc portion34 in ascending order. The four cylindrical portions 36 to 39 arearranged in order of the first cylindrical portion 36, the secondcylindrical portion 37, the third cylindrical portion 38, and the fourthcylindrical portion 39 from the inside in the radial direction. Thefirst cylindrical portion 36 and the second cylindrical portion 37extend straight in the axial direction. The third cylindrical portion 38is slightly tilted so that its radially inner part is located below itsradially outer part in the axial direction. The fourth cylindricalportion 39, which is the outermost cylindrical portion in the radialdirection, is slightly tilted so that its radially inner part is locatedabove its radially outer part in the axial direction. The upper rollerstoppers 26 are formed in the third disc portion 33. The lower rollerstoppers 27 are formed in the second disc portion 32, and the lowerroller stoppers 28 are formed in the fourth disc portion 34. The innerannular portion 23 in the radial direction includes the first discportion 31, a part of the second disc portion 32, the first cylindricalportion 36, and the second cylindrical portion 37. The outer annularportion 24 in the radial direction includes a part of the fourth discportion 34, a radially outer area bent portion 41, and projectingportions 44. The radially outer area bent portion 41 and the projectingportions 44 will be described later. Each pillar 25 includes a part ofthe second disc portion 32, the third disc portion 33, a part of thefourth disc portion 34, the third cylindrical portion 38, and the fourthcylindrical portion 39.

The cage 11 includes the radially outer area bent portion 41 that isformed by bending a radially outer area of the cage 11 inward in theradial direction. In other words, the cage 11 has the radially outerarea bent portion 41 that is formed by bending an area located radiallyoutside the pockets 21 obliquely inward in the radial direction. Theradially outer area bent portion 41 is a standing wall standing in theaxial direction. The radially outer area bent portion 41 is formed toextend continuously in an annular shape. Specifically, the radiallyouter area bent portion 41 is formed by bending the radially outer edgeof the fourth disc portion 34, which is the outermost disc portion inthe radial direction, upward in the axial direction to a predeterminedangle.

The angle of the radially outer area bent portion 41, namely the anglebetween a radially inner surface 42 of the radially outer area bentportion 41 and an upper surface 43 of the fourth disc portion 34, isshown by an angle B₁ in FIGS. 2 and 3. This angle is more than 0° andless than 45°, and is preferably 25° or more and 35° or less.

The radially outer area bent portion 41 has the projecting portions 44formed in its tip end at positions aligned with the pockets 21. That is,the projecting portions 44 are formed at positions aligned with thepockets 21 and face inward in the radial direction. The positionsaligned with the pockets 21 are such positions that the outer peripheraledges of the projecting portions 44 overlap the outer peripheral edgesof the pockets 21. That is, the radially inner edge of the radiallyouter area bent portion 41 overlaps the radially outer edges of thepockets 21.

The projecting portions 44 project inward in the radial direction beyondthe radially outer edges of the pockets 21 so as to contact the endfaces 16 of the rollers 13 accommodated in the pockets 21. Namely, theprojecting portions 44 abut on the end faces of the rollers accommodatedin the pockets 21 to restrict movement of the rollers toward the outsidein the radial direction. Specifically, the projecting portions 44 areshaped to extend continuously from the inner peripheral edge of theradially outer area bent portion 41 toward the inside in the radialdirection. That is, the radially outer area bent portion 41 and theprojecting portions 44 are formed as a single-piece member.

The projecting portions 44 are formed at circumferential positions sothat the tips of the projecting portions 44 are located in the middleparts in the circumferential direction of the pockets 21. Specifically,the projecting portions 44 are formed so that their corners 45 on thesurface 42 side, namely the innermost parts in the radial direction ofthe projecting portions 44 (the innermost corners 45 in the radialdirection of the projecting portions 44) contact the centers of the endfaces 16 of the needle rollers 13 accommodated in the pockets 21. Inthis example, the corners 45 are the corners of the projecting portions44 which are located closer to the fourth disc portion 34.

The cage 11 has three pilot holes 51, 52. The three pilot holes 51, 52serve as engagement portions for alignment. One of the pilot holes isnot shown in FIG. 1. The three pilot holes 51, 52 are formed atintervals in the circumferential direction and extend straight throughthe cage 11 in the thickness direction of the cage 11. The three pilotholes 51, 52 open in a circular shape. The three pilot holes 51, 52 areformed substantially equally spaced apart from each other. In thisexample, the three pilot holes 51, 52 are formed at intervals of 120degrees about the rotation axis 12 of the cage 11. Specifically, thepilot holes 51, 52 are formed in the middle part in the radial directionof the innermost first disc portion 31 in the radial direction. Forexample, the diameter of the pilot holes 51, 52 is φ2.5 mm or φ3 mm.

For example, the thrust roller bearing 20 having such a cage 11 includesthe plurality of needle rollers 13, the upper bearing ring 14, and thelower bearing ring 15. When the thrust roller bearing 20 is inoperation, the needle rollers 13 accommodated in the pockets 21 roll ona raceway surface 18 of the upper bearing ring 14 in the axial directionand a raceway surface 19 of the lower bearing ring 15 in the axialdirection. The cage 11 rotates around its rotation axis 12. Each of theneedle rollers 13 accommodated in the pockets 21 revolves while rotatingaround its axis. The needle rollers 13 are subjected to a radiallyoutward centrifugal force. The centers of the end faces 16 of the needlerollers 13 make sliding contact with the projecting portions 44 of thecage 11, specifically the innermost corners 45 in the radial directionof the projecting portions 44 of the cage 11. That is, the corners 45 ofthe projecting portions 44 are the areas that contact the end faces 16of the needle rollers 13.

The corners 45 have been press-flattened. The corners 45 subjected tothe press-flattening process have no sharply pointed parts and smoothlyconnect to the surfaces forming the corners 45. This reduces aggressionof the corners 45 against members that are contacted by the corners 45.

Next, a method for manufacturing the thrust roller bearing cage 11according to the embodiment of the present invention will be described.The thrust roller bearing cage 11 is manufactured by using a transferpress. The transfer press is a relatively inexpensive press machine witha less complicated configuration. FIG. 4 is a flowchart illustratingrepresentative steps of the method for manufacturing the thrust rollerbearing cage 11 according to the embodiment of the present invention.

Referring to FIG. 4, a cage material, which will later become the cage11, is first prepared (cage material preparing step: step S1). Forexample, the cage material is a thin flat steel sheet. At this stage,the cage material may be either a plate cut into a substantiallyrectangular shape or a circular plate, as the final outer shape of thecage is formed in an outer shape forming step (step S4) that will beperformed later.

Next, concave and convex portions are formed in the cage material in thethickness direction of the cage material (concave and convex portionsforming step: step S2). This ensures that the cage 11 has a large lengthdimension in the direction of its rotation axis even if the cage 11 isin the shape of a thin plate, whereby the cage 11 can appropriatelyretain the rollers.

Specifically, in this step, the cage material is subjected to a drawingprocess. In this case, the concave and convex portions can thus beformed more efficiently. FIG. 5 is a sectional view of the cage materialafter the concave and convex portions forming step. The section shown inFIG. 5 corresponds to the section shown in FIG. 2. Specifically,referring to FIG. 5, a flat plate-like cage material 56 is subjected toa drawing process to form first to fourth disc portions 61 to 64 andfirst to fourth cylindrical portions 66 to 69. A circular through bore57 is formed in the central part of the cage material 56 so as to extendtherethrough in the thickness direction. That is, in this case, the cagematerial 56 has what is called a mountain-and-valley shape made bybending the cage material 56 a plurality of times in the axialdirection.

Thereafter, pilot holes serving as engagement portions are formed (pilothole forming step: step S3). FIG. 6 is an enlarged sectional viewshowing a part of the cage material 56 after the pilot hole formingstep. The section shown in FIG. 6 corresponds to the area VI in FIG. 2.A pilot hole 71 serving as an engagement portion is formed in the middlepart in the radial direction of the first disc portion 61 so as toextend straight through the first disc portion 61 in the thicknessdirection. Three of the pilot holes 71 are formed in total so as to besubstantially equally spaced apart from each other at intervals of 120degrees in the circumferential direction.

Subsequently, the outer shape of the cage material 56 is formed (outershape forming step: step S4). FIG. 7 is an enlarged sectional viewshowing a part of the cage material 56 after the outer shape formingstep. The section shown in FIG. 7 corresponds to the area VI shown inFIG. 2, and is a section taken along line VII-VII in FIG. 8.Specifically, in this example, the cage material 56 is punched straightin the thickness direction so that the cage 11 can be formed into thefinal outer shape by a radially outer area bending step (step S7) etc.that will be performed later. In this case, the outer shape of the cagematerial 56 can be formed relatively easily and accurately. A radiallyouter edge 72 of the cage 11, specifically, a radially outer edge 72 ofthe fourth disc portion 64, is thus formed.

When forming the outer shape, the cage material 56 is punched so as toform portions that will later become projecting portions 70. That is, inthis case, the outer shape forming step is also a projecting portionsforming step, i.e., the step of forming the projecting portions. FIG. 8shows a part of the cage material 56 after a pocket forming step, whichis the step following the outer shape forming step. FIG. 8 correspondsto FIG. 1. When punching the cage material 56 so as to form theprojecting portions 70, the cage material 56 is aligned in thecircumferential direction by using the plurality of pilot holes 71.Specifically, a plurality of guide pins (not shown), which serve as whatis called pencil-like alignment jigs having a pointed end and having atapered shape with its diameter gradually increasing from the pointedend, are prepared and are gradually inserted into the plurality of pilotholes 71 from one side in the thickness direction.

The cage material 56 is thus aligned by using the plurality of guidepins and is punched into the overall outer shape with a punching machine(not shown) in view of the positions, the shape, etc. of the projectingportions 70. Accordingly, even if the cage material 56 is slightly outof alignment with the punching machine with respect to the properpositions where the projecting portions 70 are supposed to be formed,the cage material 56 can be aligned with the punching machine withrespect to the proper positions where the projecting portions 70 aresupposed to be formed, as the pencil-like guide pins having a pointedend are gradually inserted into the pilot holes 71. The punching processcan thus be performed. In this example, since the cage material 56 hasthe three pilot holes 71, rotation etc. of the cage material 56 isprevented during alignment. The cage material 56 can thus be alignedmore accurately.

Subsequently, pockets are formed (pocket forming step: step S5). FIG. 9is an enlarged sectional view showing a part of the cage material afterthe pocket forming step. The section shown in FIG. 9 corresponds to thearea III shown in FIG. 2 and is a section taken along line IX-IX in FIG.8. In this example, each pocket 73 is punched out along a part of thesecond disc portion 62, the third disc portion 63, and a part of thefourth disc portion 64 and also along the third cylindrical portion 68and the fourth cylindrical portion 69 so as to extend straight throughthe cage material 56 in the thickness direction. Although not shown inFIG. 9, the upper roller stoppers and the lower roller stoppers, whichare shaped so as to project into the pockets 73 in the circumferentialdirection, are formed simultaneously with the pockets 73. That is, thepockets 73 are punched out in view of the shape of the upper rollerstoppers and the lower roller stoppers so as to conform to the outershape of needle rollers 13 that are to be accommodated in the pockets73. The plurality of pockets 73 may be punched out either all at once orone by one.

When forming the pockets 73 in the cage material 56, the pilot holes 71are also used to align the cage material 56 to be punched with apunching machine (not shown) for punching out the pockets. That is, thepockets 73 are formed with respect to the positions of the pilot holes71. As in the case of the outer shape forming step, alignment in thecircumferential direction is performed by using the plurality of pilotholes 71. Specifically, a plurality of guide pins serving as sharppencil-like alignment jigs are prepared and the tip ends of the guidepins are gradually inserted into the plurality of pilot holes 71 fromone side in the thickness direction as described above. The cagematerial 56 is thus aligned by using the plurality of guide pins, andthe pockets 73 are punched out with the punching machine in view of thepositions, shape, etc. of the pockets 73. The pockets 73 are thus formedin phase with the projecting portions 70 in the circumferentialdirection, so that an appropriate positional relationship can beestablished between the pockets 73 and the projecting portions 70.Accordingly, the projecting portions 70 can be accurately andefficiently formed in terms of the positional relationship of theprojecting portions 70 with the pockets 73 to be formed. Since theprojecting portions 44 are accurately formed at the appropriatepositions, end faces 16 of the needle rollers 13 can appropriatelycontact the projecting portions 44 when the bearing is in operation. Theplurality of pockets 73 may be punched out either all at once or one byone.

In the present embodiment, the pilot holes 71 are formed in an arealocated radially inside the pockets 73. In this case, the pilot holes 71can be formed by making effective use of the available area of the cage11.

In the present embodiment, the pilot holes 71 are formed so as not tooverlap the pockets 73 in the circumferential direction. This can avoidlocal strength reduction in the circumferential direction of the cage11. The positional relationship of the pockets 73 with the pilot holes71 can be determined as desired. Specifically, in this example, theplurality of pockets 73 are formed so that each of the pilot holes 71 islocated at a position corresponding to the middle in the circumferentialdirection between adjoining ones of the pockets 73.

Subsequently, as shown in FIGS. 8 and 9, an annular groove 79 is formedat a position radially outside the pockets 73 in the cage material(groove forming step: step S6). In this step (step S6), the groove 79 isformed at such a position that the cage material is to be bent along thegroove 79 when forming a radially outer area bent portion 41 in theradially outer area bending step (step S7) which will be describedbelow. Although performing the groove forming step (step S6) makes iteasier to bend a radially outer area of the cage material 56 inward inthe radially outer area bending step (step S7) described below, thegroove forming step (step S6) may be omitted. The steps S4 to S6 may beperformed in any order.

Subsequently, an area of the cage material 56 which is located radiallyoutside the pockets 73 is bent inward in the radial direction to a tiltangle of 45° to form a radially outer area bent portion (radially outerarea bending step) (step S7). In the case where the groove forming step(step S6) is performed, a radially outer area of the cage material 56 isbent along the groove 79 to form the radially outer area bent portion.

FIG. 10 is an enlarged sectional view showing a part of the cagematerial during the radially outer area bending step. FIGS. 11 and 12are enlarged sectional views illustrating how the radially outer areabending step is performed. FIGS. 13 and 14 are enlarged sectional viewsshowing a part of the cage material 56 after the radially outer areabending step. The sections shown in FIGS. 10 and 13 correspond to thearea VI in FIG. 2. The section shown in FIG. 14 corresponds to the areaIII in FIG. 2. The sections shown in FIGS. 11 and 12 show the positionalrelationship of an area of the cage material 56 which is locatedradially outside the position corresponding to the area VI in FIG. 2with holding members 101, 102 and a pressing member 103. In thisexample, as shown in FIG. 10, the annular radially outer edge 72 of thecage material 56 is first bent along the entire circumference so as toextend straight in the thickness direction. That is, the angle B₂between a radially inner surface 75 of a radially outer area bentportion 74 and an upper surface 76 of the fourth disc portion 64 isapproximately a right angle. For example, the radially outer edge 72 isbent to a right angle by the following method, although the presentinvention is not particularly limited to this method. As shown in FIG.11, the entire cage material 56 except for a radially outer area of thefourth disc portion 64 is sandwiched between the holding members 101,102 in the vertical direction and is held therebetween, and the pressingmember 103 is placed under the radially outer area of the fourth discportion 64. As shown in FIG. 12, the pressing member 103 is then movedupward. The radially outer area bent portion 74 can thus be formed at aright angle with respect to the fourth disc portion 64.

Subsequently, as shown in FIGS. 13 and 14, the radially outer area bentportion 74 is tilted further inward in the radial direction, whereby theradially outer area bent portion 74 tilted inward in the radialdirection to a tilt angle of less than 45° is formed. In this case, thebending angle (tilt angle), that is, the angle between the radiallyinner surface 75 of the radially outer area bent portion 74 and theupper surface 76 of the fourth disc portion 64 is shown by an angle B₃in FIGS. 13 and 14. The angle B₃ corresponds to the angle B₁ describedabove. In other words, the angle B₁ is equal to the angle B₃. The anglesB₁, B₃ are more than 0° and less than 45°, and are preferably 25° ormore and 35° or less.

In this example, in terms of the positional relationship in thecircumferential direction, the projecting portions 70 are formed at thepositions corresponding to the middle parts in the circumferentialdirection of the pockets 73. The projecting portions 70 are thus formedat appropriate positions. Specifically, the projecting portions 70 abuton the centers of the end faces 16 of the needle rollers 13 at theircorners 77 located closer to the fourth disc portion 64. Finally, theareas of the projecting portions 70 which are to contact the end faces16 of the needle rollers 13 are subjected to a press-flattening process.The thrust roller bearing cage 11 configured as shown in FIGS. 1 to 3 isthus manufactured.

The step of tilting the radially outer edge 72 of the cage material 56inward in the radial direction after the annular radially outer edge 72is bent so as to extend straight in the thickness direction and thepress-flattening process may be performed successively. FIG. 15 is anenlarged sectional view illustrating how the radially outer area bendingstep is performed. FIG. 16 is an enlarged sectional view illustratinghow the press-flattening step is performed. FIG. 17 is an enlargedsectional view showing the tip of the radially outer area bent portionafter the press-flattening step. Specifically, as shown in FIG. 15,after the cage material 56 is bent so that the radially outer area bentportion 74 extends at a right angle with respect to the fourth discportion 64, an area of the cage material 56 which is located radiallyinside the radially outer area bent portion 74 is sandwiched betweendies 104, 105 in the vertical direction and is held therebetween. Atthis time, the radially outer edge of the upper die 104 is locatedradially inside the radially outer edge of the lower die 105. A die 106that presses the radially outer area bent portion 74 downward from aboveis also placed so as to contact a radially outer surface 78 of theradially outer area bent portion 74. The die 106 includes a radiallyinner end face 106 a that faces the upper die 104 and extends in thevertical direction, a horizontal face 106 b that is continuous with theradially inner end face 106 a and extends outward in the radialdirection, and a radially inner face 106 c that faces the radially outersurface 78 of the radially outer area bent portion 74 and extends in thevertical direction. A part 106 d of the die 106 where the horizontalface 106 b and the radially inner face 106 c meet has a round (R) shape.When the die 106 is moved downward so that the radially inner end face106 a moves along a radially outer end face 104 a of the die 104, theradially outer area bent portion 74 can be tilted inward in the radialdirection as guided by the round part 106 d. Subsequently, as shown inFIG. 16, the die 104 is moved further downward so that the radiallyinner corner of the radially outer area bent portion 74 is smoothed bythe radially outer end face 104 a of the die 104 and the radially outercorner of the radially outer area bent portion 74 is smoothed by thehorizontal face 106 b of the die 106. As shown in FIG. 17, theprojecting portions 70 subjected to the press-flattening process in theareas of the projecting portions 70 which are to contact the end facesof the rollers can be formed in this manner.

In the above embodiment, the corners of the projecting portions whichare located closer to the fourth disc portion contact the centers of theend faces of the needle rollers accommodated in the pockets. However,the present invention may have the following configuration. FIG. 18 is asectional view showing a part of a cage having this configuration. FIG.18 corresponds to the section shown in FIG. 3.

Referring to FIG. 18, a thrust roller bearing cage 81 according toanother embodiment of the present invention has a radially outer areabent portion 82 having projecting portions 83 formed at positionscorresponding to the positions of pockets 85. The projecting portions 83contact the centers of end faces 16 of needle rollers 13 accommodated inthe pockets 85 at corners 84 located on the opposite side of theprojecting portions 83 from a fourth disc portion 86. The corners 84have been press-flattened. This configuration can be achieved bymachining the corners 84 with a jig angled so as to conform to thecorner 84 in the radially outer area bending step.

A pilot hole may be formed at a position where a pocket is supposed tobe formed. In other words, one of the plurality of pockets may be usedas a pilot hole. FIG. 19 shows a part of a cage having thisconfiguration. Referring to FIG. 19, a thrust roller bearing cage 91according to still another embodiment of the present invention includesa plurality of pockets 92 and pillars 93 each located between adjoiningtwo of the pockets 92. A pilot hole 95 is formed at a position where apocket 92 is supposed to be formed in a pillar 94 located between thepockets 92. In this configuration, one of the plurality of pockets 92that are formed equally spaced apart from each other in thecircumferential direction is replaced with this pilot hole 95.

In the above embodiment, the pilot holes extend straight through thecage in the thickness direction. However, the present invention is notlimited to this. For example, the pilot holes extending through the cagemay have a tapered wall surface. The pilot holes are not limited to thecircular holes and may be quadrilateral holes, triangular holes, etc.The pilot holes are formed as engagement portions. However, the presentinvention is not limited to this. The engagement portions may have otherconfigurations. For example, the engagement portions may be formed bycutouts.

In the above embodiment, a drawing process is performed in the concaveand convex portions forming step. However, the present invention is notlimited to this. A process other than the drawing process, such as abending process, may be used to form concave and convex portions.

In the above embodiment, the cage has the concave and convex portionswhich are formed in the thickness direction. However, the presentinvention is not limited to this. The cage may not have the concave andconvex portions which are formed in the thickness direction, and a cagein the form of what is called a laminate of two plates may be used.

In the above embodiment, the thrust roller bearing having such a cage asdescribed above may not have bearing rings. Rollers other than theneedle rollers, such as long rollers etc., may be used.

Advantageous effects of the thrust roller bearing cage of the presentembodiment and the method for manufacturing the same will be describedbelow. For thrust roller bearing cages having a radially outer area bentportion formed by bending an area located radially outside pocketsinward in the radial direction to various tilt angles θ, the inventorsfound out the following as shown in Table 1 regarding resistance toradial load, the outside diameter size of the cage, and characteristicsof running torque after long term use.

TABLE 1 Comparative Exam- Exam- Comparative Comparative ComparativeExample 1 Example 1 ple 2 Example 3 ple 4 Example 5 Example 6 Example 7Example 2 Example 3 Example 4 Tilt Angle θ 0 10 15 20 25 30 35 40 45 5055 Radial Load ⊚ ⊚ ⊚ ◯ ◯ ◯ ◯ Δ X X X Outside X Δ Δ Δ ◯ ◯ ◯ ⊚ ⊚ ⊚ ⊚Diameter Torque after ◯ Δ Δ Δ ◯ ◯ ◯ ◯ ◯ Δ X Long-Term Use

In Table 1, the radial load refers to resistance of projecting portionsto a rotational centrifugal force of rollers in a thrust roller bearingaccommodating the rollers in pockets of a cage. In the table, “⊚” meansthat the shape of the projecting portions can be maintained when therotational centrifugal force of the rollers is very large, “◯” meansthat the shape of the projecting portions can be maintained when therotational centrifugal force of the rollers is large, “Δ” means that theshape of the projecting portions can be maintained when the rotationalcentrifugal force of the rollers is moderate (as large as a rotationalcentrifugal force of rollers of commonly used thrust roller bearings),and “×” means that the shape of the projecting portions cannot bemaintained when the rotational centrifugal force of the rollers ismoderate. That is, “⊚” means the highest resistance to radial load,followed by “◯,” “Δ,” and “×” in descending order.

The outside diameter size refers to the magnitude of the outsidediameter of the cage in the case where the positions and the shape ofthe pockets of the cage are the same. In the table, “×” means theoutside diameter in the case where the tilt angle is 0°, “Δ” means thatthe outside diameter is slightly smaller than that in the case where thetilt angle is 0°, “◯” means that the outside diameter is smaller thanthat in the case where the tilt angle is 0°, and “⊚” means that theoutside diameter is significantly smaller than that in the case wherethe tilt angle is 0°. That is, “⊚” means the smallest outside diameter,followed by “◯,” “Δ,” and “×” in ascending order.

The torque after long term use refers to running torque in the casewhere the thrust roller bearing accommodating the rollers in the pocketsof the cage has been used for a long period of time. In the table, “◯”means that the running torque after long term use shows little decreasefrom initial running torque, “Δ” means that the running torque afterlong term use is slightly lower than the initial running torque, and “×”means that the running torque after long term use is significantly lowerthan the initial running torque. That is, “◯” means the smallestdecrease from the initial running torque, followed by “Δ” and “×” inascending order.

As shown in Table 1, in Examples 1 to 7 having a bent portion formed bybending the area located radially outside the pockets inward in theradial direction to a tilt angle of less than 45°, when the projectingportions are subjected to a rotational centrifugal force of the rollers(a force applied from the inside toward the outside in the radialdirection), the projecting portions can withstand the load from therollers and the shape of the projecting portions tends to be maintained.Accordingly, resistance to the radial load that is applied duringrotation of the thrust roller bearing can be maintained. In terms of theresistance to the radial load, the tilt angle θ is preferably 40° orless, and more preferably 35° or less. In Comparative Examples 1 to 3having a tilt angle of 45° or more, when the projecting portions aresubjected to the rotational centrifugal force of the rollers, the bentportion is lifted so as to increase the tilt angle (toward 90°), andresistance to the radial load cannot be maintained.

In Examples 1 to 7, the radially outer area bent portion is formed bybending the area located radially outside the pockets inward in theradial direction to a tilt angle of less than 45°. Accordingly, theprojecting portions have a smaller contact area with the rollers than inthe case where the tilt angle is 0° (Comparative Example 1) as in PatentLiterature 1. Specifically, in Patent Literature 1 in which the tiltangle is 0°, the entire tip end faces of the projecting portions contactthe central parts of the end faces of the rollers accommodated in thepockets. On the other hand, in Examples 1 to 7, the innermost corners inthe radial direction of the tip end faces of the projecting portionscontact the centers of the end faces of the rollers accommodated in thepockets. This contact is close to point contact rather than surfacecontact. When the rollers are biased from the central axis of the cagetoward the outside in the radial direction by the rotational centrifugalforce of the rollers, friction is generated between each roller and thepart of each projecting portion which contacts the roller. Accordingly,the projecting portions of Examples 1 to 7 having a smaller contact areawith the rollers can reduce rotational resistance, whereby runningtorque can be reduced.

As described above, in the thrust roller bearing cage of the presentembodiment and the method for manufacturing the same, the radially outerarea bent portion is formed by bending the area located radially outsidethe pockets inward in the radial direction to a tilt angle θ of lessthan 45°. Accordingly, running torque can be reduced, and resistance toa radial load that is applied during rotation of the thrust rollerbearing can be maintained.

Since the tilt angle θ is less than 45°, it is easy to adjust the heightof the radially outer area bent portion. In this case, the projectingportions can be easily made to contact the centers of the end faces ofthe rollers. Even if the parts of the projecting portions which contactthe rollers become worn, contact between the projecting portions and thecenters of the end faces of the rollers tends to be maintained. InPatent Literature 1 in which the tilt angle is 0°, contact with therollers depends on the thickness of the cage.

As shown in Table 1, the outside diameter can be reduced by setting thetilt angle θ to 25° or more. The proportion of the areas that supportthe rollers to the entire cage can thus be increased.

As shown in Table 1, in Examples 1 to 7 in which the tilt angle is lessthan 45°, even if the parts of the projecting portions which contact therollers become worn after long term use of the thrust roller bearing,the area of the parts of the projecting portions which contact therollers (the contact area of the worn projecting portions with therollers) increases only slightly. A decrease in running torque cantherefore be restrained. If the projecting portions become worn afterlong term use of the cage, the contact area of the projecting portionswith the rollers increases, but the rate of increase is low, andtherefore a decrease in running torque after long term use can berestrained. In view of this, the tilt angle θ is preferably 25° or moreand 40° or less.

As shown in Table 1, when the tilt angle θ is 25° or more and 35° orless, all of the radial load, the outside diameter size, and the torqueafter long term use are evaluated as “◯.” That is, setting the tiltangle θ to 25° or more and 35° or less can restrain a decrease inresistance to radial load, an increase in size, and a decrease inrunning torque after long term use.

The embodiments disclosed herein are by way of example in all respectsand should not be interpreted as restrictive. The scope of the presentinvention is defined by the claims rather than by the above embodiments,and the invention is intended to cover all changes and modificationswithin the spirit and scope of the invention as defined by the claims.

INDUSTRIAL APPLICABILITY

The thrust roller bearing cage according to the present invention andthe method for manufacturing the same are effectively utilized to meetdemands for thrust roller bearing cages with excellent performance andmore efficient methods for manufacturing such a thrust roller bearingcage.

REFERENCE SIGNS LIST

-   -   11, 81, 91 Cage    -   12 Rotation Axis    -   13 Roller    -   14, 15 Bearing Ring    -   16, 17 End Face    -   18, 19 Raceway Surface    -   20 Thrust Roller Bearing    -   21, 73, 85, 92 Pocket    -   22, 57 Through Bore    -   23, 24 Annular Portion    -   25, 93, 94 Pillar    -   26, 27, 28 Roller Stopper    -   31, 32, 33, 34, 61, 62, 63, 64, 86 Disc Portion    -   36, 37, 38, 39, 66, 67, 68, 69 Cylindrical Portion    -   41, 74, 82 Radially Outer Area Bent Portion    -   42, 43, 75, 76, 78 Surface    -   44, 70, 83 Projecting Portion    -   45, 77, 84 Corner    -   51, 52, 71, 95 Pilot Hole    -   56 Cage Material    -   72 Edge    -   79 Groove    -   101, 102 Holding Member    -   103 Pressing Member    -   104, 105, 106 Die    -   104 a Radially Outer End Face    -   106 a Radially Inner End Face    -   106 b Horizontal Face    -   106 c Radially Inner Face    -   106 d Part

1. A thrust roller bearing cage included in a thrust roller bearing andincluding a plurality of pockets accommodating rollers, comprising: aradially outer area bent portion formed by bending an area locatedradially outside the pockets inward in a radial direction to a tiltangle of less than 45°; and projecting portions that are formed in a tipend of the radially outer area bent portion at positions aligned withthe pockets and project inward in the radial direction beyond radiallyouter edges of the pockets so as to contact end faces of the rollersaccommodated in the pockets.
 2. The thrust roller bearing cage accordingto claim 1, wherein the tilt angle is 25° or more and 35° or less. 3.The thrust roller bearing cage according to claim 1, wherein areas ofthe projecting portions which are to contact the end faces of therollers are subjected to a press-flattening process.
 4. A method formanufacturing a thrust roller bearing cage included in a thrust rollerbearing and including a plurality of pockets accommodating rollers,comprising the steps of: preparing a cage material that will laterbecome the cage; forming an outer shape of the cage material so that theouter shape has portions that will later become projecting portionsprojecting inward in a radial direction beyond radially outer edges ofthe pockets so as to contact end faces of the rollers accommodated inthe pockets; forming the pockets in the cage material; and forming aradially outer area bent portion by bending an area of the cage materialwhich is located radially outside the pockets inward in the radialdirection to a tilt angle of less than 45°.
 5. The method formanufacturing the thrust roller bearing cage according to claim 4,wherein in the step of forming the radially outer area bent portion, thearea of the cage material which is located radially outside the pocketsis bent inward in the radial direction to a tilt angle of 25° or moreand 35° or less.
 6. The method for manufacturing the thrust rollerbearing cage according to claim 4, further comprising the step of:press-flattening areas of the projecting portions which are to contactthe end faces of the rollers.